ESI stopped working (tag not flusheable)

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I have a setup working for the last 4 year which no problems. Basically we are dropping esi-fragments for price-information into a wordpress-pages and posts. basically all prices are tagged with "tag_price" and more generic tags like "tag_price_beamer" or "tag_price_tvset".
When never prices are availiable we drop/flush the tag. Later a (self-brewed) crawler will tick the pages for to refresh esi-fragments in the pages.
The same procedure was running for about 4 year. From a couple of days ago we recorded to following issues:

- the flush of the tag seems not to invalidate the esi-fragment cache
- if the entire ls-cache is flushed the data is refreshed
- just flushing a single page-url will not work

- additionally the crawlers runs are 6-10times longer and produce a lot of more load on the server.

The seems to be entirely related with the 7.x release. The same servers runs the stage- but the prod-instances. The ls-cache most updated first on the stage-instance. Whereas the problem started on the prod-instance BEFORE this instance was updated.

This there a way to clearly check within a html-page if esi is working ? Or can I check somehow if and how many fragments are in the litespeed esi-fragment cache. We are using REDIS for the object cache; are the esi-fragments also stored in REDIS ?

thx for your help

Start by reproducing the ESI tag flush, single-page flush, and full LiteSpeed cache flush described in the report, comparing production and staging behavior. Check whether ESI fragments are invalidated and whether crawler runtime and server load return to normal; the report does not identify specific files or tests.

Written by the indexing model from the issue text.