Extension tab/modules missing after phpBB auto update

First reported here: https://www.phpbb.com/customise/db/extension/pages/support/topic/133251

It seems that an extension using its own acl options for its ACP module (for example pages and board rules, which use acl_a_pages and acl_a_boardrules) can vanish from the ACP, apparently due to stale or incorrect caching of permissions, after an automatic update to phpBB.

To reproduce:
Start with a fresh install of 3.0.12
Update it to 3.1.1
Install an extension (Pages or Board Rules)
Update phpBB to 3.1.2
Look for the missing Extensions tab
Do a cache purge and the Extensions tab returns on page reload.

I do not know why this is happening only when starting from 3.0.12. Starting from 3.1.1 does not reproduce this issue.

Looking at the cache, it appears that after auto-updating the board the ext_finder cache file is missing a lot of data compared to after re-purging the cache.

So it appears that after an automatic board update, either the cache is not being purged, or it is being purged when the user does not have correct permissions, or something. Either way, after the auto-update, basically the extensions that have their own auth acls are not being recognized by the user as having permissions until the user manually purges the cache.

Other bizarre behaviors:
1. Trying this with both Pages and Boardrules installed, after update only 1 vanished but the other was visible.
2. Trying this with other Extensions tab extensions installed (using acl_a_board as their permission) and both Pages and Boardrules remained visible, and nothing vanished.