Debian 11下apt full-upgrade无法升级可升级的unifi包问题求助

我目前在用Debian 11 Bullseye（还没来得及升级到Bookworm），执行apt update之后，系统提示有1个包可以升级，让我用apt list --upgradable查看具体内容。

执行apt list --upgradable -a后，显示是unifi包有新版本可以升级：

$ sudo apt list --upgradable -a

Listing... Done

unifi/stable 7.5.176-22707-1 all [upgradable from: 7.4.162-21057-1]

unifi/now 7.4.162-21057-1 all [installed,upgradable to: 7.5.176-22707-1]

这是个第三方包，但我觉得这不应该影响升级流程。我尝试用apt full-upgrade -a来升级所有可升级包，结果却显示没有包需要处理：

$ sudo apt full-upgrade

Reading package lists... Done

Building dependency tree... Done

Reading state information... Done

Calculating upgrade... Done

0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

我查了apt policy unifi的信息，候选版本确实是那个新版本：

$ apt policy unifi

unifi:

Installed: 7.4.162-21057-1

Candidate: 7.5.176-22707-1

Version table:

7.5.176-22707-1 500

500 http://www.ubnt.com/downloads/unifi/debian stable/ubiquiti amd64 Packages

*** 7.4.162-21057-1 100

100 /var/lib/dpkg/status

而且这个包也没有被标记为保留状态：

$ dpkg --get-selections | grep unifi

unifi                                           install

我知道直接用apt install unifi应该能尝试升级，但就是搞不懂为什么apt full-upgrade会直接忽略这个可升级的包。

编辑补充：
按照@A.B的建议，我尝试执行预安装（只下载包不安装）的命令，终于找到问题所在了：

$ sudo apt install -d unifi

Reading package lists... Done

Building dependency tree... Done

Reading state information... Done

Some packages could not be installed. This may mean that you have

requested an impossible situation or if you are using the unstable

distribution that some required packages have not yet been created

or been moved out of Incoming.

The following information may help to resolve the situation:

The following packages have unmet dependencies:

unifi : Depends: mongodb-server (>= 1:3.6.0) but 1:3.2.11-2+deb9u1 is to be installed or

mongodb-10gen (>= 3.6.0) but it is not installable or

mongodb-org-server (>= 3.6.0) but it is not installable

E: Unable to correct problems, you have held broken packages.

我其实挺希望apt full-upgrade能直接告诉我依赖有问题，而不是默默忽略这个包，不过现在总算搞清楚原因了。

备注：内容来源于stack exchange，提问作者Roel Schroeven