MariaDB THREADS_CONNECTED build up with static php-fpm

Question

Under what circumstance does php-fpm maintain a DB connection beyond the end of a script or request?

The Problem

• My app is written in PHP, on top of Laravel 8.
• Every hour, I have to restart php-fpm to prevent the MariaDB database from hitting max_connections = 150 and disabling the app since no more connections can be created.

Diagnostic Information

• PHP-FPM is configured as static, with a max child count of 39.
• DB connections are not configured as persistent.
• Raising max_connections above 150 only delays the issue.
• There are three DB nodes and three app nodes. The app nodes only talk to their partner DB node in the same region.
• The DB nodes are replicating to each other via Galera.
• The DB nodes have too many connections independently, not as a cluster.
• Checking show full processlist shows me that the vast majority of connections are in SLEEP state and doing nothing.
• Using the remote port from the processlist and ss on the app node as well as the php-fpm status page, I've determined that the children holding the connections open are themselves in idle state.

Attempted solutions

• I've switched php-fpm to dynamic and set the idle-timout to 10s. The children do not quit, and I can't see any errors.
• I've turned down the number of requests a php-child can handle before it is reaped from 100 to 1 with no effect.
• I've registered a shutdown handler with PHP that checks if my DB connection is open and produces an alert. No alerts have been sent.
• I've set up a cronjob to systemctl restart php7.4-fpm every hour. This alleviates the issue, but obviously isn't a good solution.

Thanks for reading and any idea that might help. I've also asked this same question on StackOverflow