Search API Schema reverts back to 4.2 after deploys or randomly

Ah, I'm so glad you posted this. One of our clients has had this same issue this week, having followed the steps outlined in a Search API Solr issue previously: https://www.drupal.org/project/search_api_solr/issues/3401710 💬 Updating to 4.3 Errors Fixed

It would be good to know what the cause is and when there may be a fix.

Update: one of our sites randomly reverted back to 4.2.10 on 3/1 even though there was no deploy. This is a huge problem as it prevents new items from being indexed and there's no notice or warning other than our customers reporting missing content.

Since this causes a reduction in service for clients, and can happen randomly, I'm raising the priority to "Major".

Happy to help out testing any patches.

I've been communicating with Pantheon's support staff and they've said:

Good day, for an update, Pantheon Platform Engineers already released an update to the Search API Pantheon module https://www.drupal.org/project/search_api_pantheon → just today. The latest version 8.1.7 contains fixes for the Sticky Solr Schema.

I'll be installing this and monitoring on site asap.

Just a quick heads up, we've released two versions (8.1.7 and 8.1.8) which resolve bugs that caused this issue. Let us know if you still see it after updates!

Bumping this up, this is still happening as it seems the default Solr Schema template of Pantheon containers is using Solr Schema 4.2 and that's the reason why creating a new multidev environment is installed with Solr Schema 4.2 instead of the Solr Schema provided by the search_api_solr module.

Not sure if there is a plan to update the default Solr Schema, or at least fix the reposting of Solr Schema.

We can confirm the Solr Schema on Pantheon (all environments) is 4.2.10 with search_api_pantheon 8.1.8, search_api 8.x-1.34, and search_api_solr 4.3.3

/admin/config/search/search-api/server/pantheon_solr8 reports the following:
Schema Version 4.2.10
Schema drupal-4.2.10-solr-8.x-1
Minimal required schema version 4.3.0
Preferred schema version 4.3.3
Configured Solr Version 8.0.0
Detected Solr Version 8.11.2

Per #5 our Solr Schema did not revert from 4.3.1 AFAIK it has been 4.2.10 since we configured our search. Our new content is being indexed on creation. We do have synonyms defined however and those are not working as expected.

To echo what @bsnodgrass mentioned in #11, I did quite a bit of research trying to find what, if anything, was causing the schema to not get fully updated.
I posted most of my findings on https://www.drupal.org/project/search_api_pantheon/issues/3449701 💬 Support for Configuring Synonyms Active , which seems to be related, but the TL;DR is that it appears there is a hidden schema somewhere on the Pantheon platform that is not being overridden by the custom schema that is posted. For more details on my findings, please check the linked issue.

Until we have access to be able to update the actual config, I don't believe this issue will be solved.

per @danreb on https://www.drupal.org/project/search_api_pantheon/issues/3449701#commen... 💬 Support for Configuring Synonyms Active

He confirmed "The default Solr schema in Pantheon platform was set to 4.2.10, currently reposting of Solr Schema on Pantheon was broken (Sticky Solr Schema bug)"

and recommended. "If you want your custom config to take affect, what you need to do right now is to open a ticket and let the CSE or the platform engineers reposted the config for you in the affected environments."