Rails/React应用部署Heroku时CSS文件致构建失败求助

• 我的Rails/React应用部署到Heroku时，只要导入某一个特定CSS文件就会构建失败，注释掉该文件的导入语句后，构建就能正常完成，这个CSS是唯一引发问题的文件。
• 尝试移除了CSS里的:root自定义变量，把所有变量引用硬编码成具体值，问题仍然存在。
• 执行npm run heroku-postbuild时出现如下错误：

Creating an optimized production build...
Failed to compile.

TypeError: Cannot read properties of undefined (reading 'toLowerCase')
    at Array.some (<anonymous>)
    at runMicrotasks (<anonymous>)

我的代码里并没有使用toLowerCase()方法，完全找不到问题的来源。目前只能通过逐个注释CSS代码块来排查，但希望能快速定位并解决问题。

引发问题的CSS代码： #

:root {
  --font-size-a: 1.25rem;
  --font-size-b: 0.9rem;
  --font-size-c: 0.5rem;
  --font-size-d: 0.35rem;
  --scoresheet-border: 2px solid #222;
}

[data-font-size="A"] {
  font-size: var(--font-size-a);
}

[data-font-size="B"] {
  font-size: var(--font-size-b);
  line-height: 80%;
}

[data-font-size="C"] {
  font-size: var(--font-size-c);
}

[data-font-size="D"] {
  font-size: var(--font-size-d);
  letter-spacing: -1px;
  line-height: 80%;
}

.scoresheet-container {
  border: 4px solid #333;
  background-color: #e4e4e4;
  max-width: min-content;
  padding: 0 1rem 1rem;
  position: relative;
}

.scoresheet-container .scoresheet-top {
  width: 100%;
  display: inline-flex;
  padding: 0.5rem 0.75rem;
}
.scoresheet-container .scoresheet-top .player-name {
  flex-basis: 100%;
  display: flex;
  gap: 0.5rem;
}

.scoresheet-container .scoresheet-top .player-name h5 {
  align-self: flex-end;
  margin-bottom: 0.5rem;
}

.scoresheet-container .scoresheet-top .player-name-slot {
  flex-basis: 80%;
  border-bottom: 2px solid black;
  display: flex;
  place-items: center;
  justify-content: center;
}

.scoresheet-container .scoresheet-top .player-name-slot p {
  font-size: clamp(1rem, 2rem);
  font-weight: bold;
}

.scoresheet[data-layout="horizontal"] {
  border: var(--scoresheet-border);
  display: flex;
  align-items: flex-start;
  flex-direction: row;
  gap: 0.25rem;
}

.scoresheet[data-layout="vertical"] {
  border: var(--scoresheet-border);
  display: flex;
  /* align-items: flex-start; */
  flex-direction: column;
  gap: 0.75rem;
}

.scoresheet-section {
  width: 400px;
  font-weight: bold;
  display: grid;
  grid-template-rows: 1fr;
  display: flex;
  flex-direction: column;
  margin: 2px;
  border: var(--scoresheet-border);
}

.scoresheet-section header {
  border-bottom: var(--scoresheet-border);
}

.scoresheet-section-header .header-cell:not(:first-child) {
  justify-content: center;
}

.scoresheet-section-header {
  background-color: #666;
  color: #fff;
}

.scoresheet-row {
  display: flex;
  max-height: 2rem;
  justify-content: center;
}

.scoresheet-row :first-child {
  padding-left: 0.25rem;
}

.scoresheet-row:not(:last-of-type) {
  border-bottom: var(--scoresheet-border);
}

.scoresheet-row > * {
  display: flex;
  justify-content: center;
  align-items: center;
}

.scoresheet-row .header-cell {
  justify-content: flex-start;
}

.scoresheet-row > :not(:last-child) {
  border-right: var(--scoresheet-border);
  /* background-color: green; */
}

.scoring-option-label {
  flex-basis: 45%;
}

.how-to-score {
  flex-basis: 30%;
  text-align: center;
}

.how-to-score:not(.header-cell) {
  line-height: 12px;
}

.points-scored {
  /* flex-grow: 1; */
  flex-basis: 25%;
}

.points-scored.total {
  background-color: #ccc;
}

.scoring-option .points-scored:not(.locked):hover {
  cursor: pointer;
}

.points-scored.selected {
  background-color: #b52717;
  color: #eee;
}

svg.right-arrow {
  max-height: 1.5rem;
  max-width: min-content;
}

.scoring-option-label img {
  margin-left: auto;
  margin-right: 1rem;
  border: 1px solid black;
  border-radius: 8%;
}

/* .scoresheet-row p {
  margin-right: 0.5rem;
} */

.header-cell.scoresheet-row > p {
  margin: auto 0;
}

.scoresheet-row p ~ span {
  text-align: center;
}

.scoresheet-cell {
  display: flex;
  gap: min(10%, 0.5rem);
}

.capitalize {
  text-transform: capitalize;
}

.uppercase {
  text-transform: uppercase;
}

#takeScoreMessage {
  display: flex;
  place-items: center;
  gap: 0.75rem;
  position: absolute;
  top: 40%;
  left: 103%;
  font-size: 2rem;
  font-family: "Righteous", cursive;
  font-weight: bold;
  color: goldenrod;
  text-shadow: -1px 2px 2px #333;
  /* bottom: 0; */
  /* visibility: visible; */
}

内容的提问来源于stack exchange，提问作者FlexOne