Why HTML updates but CSS don’t follow on my WordPress site?

Before posting here, I tried pretty much everything I could base on the knowledge I have on WordPress to fix this issue. I contacted technical support, I asked for help from my hosting service, but nothing came out of it. So I came here to ask you for help, cause I’m losing my mind on this!

Right, so here’s my problem. I had to make a website for work using WordPress, which I did, using a plugin called PostX to display beautiful news items and titles. I guess what it does is add fancy CSS to some < div> and <h*> titles (but I have no idea how it actually works). Now, the problem is, some random day, as I was modifying the home page to add some text and published it, all the CSS of the plugin’s blocks broke.

* Here is what the page looked like (the french version is not broken as it was not edited): https://inseedev.nexen.net/DMCSI2022/fr/accueil/
* Here is what it looks like after editing: https://inseedev.nexen.net/DMCSI2022/

You may think that what I modified on the page is responsible for the CSS to not be applied anymore on this specific page, but here’s the thing: wherever I just press the button “Edit page” on my site, on any page, every element of the Plugin break on this page. Even if I don’t edit a damn thing, but just publish right away. Even weirder: the CSS is alright in the embedded editor preview, but it still breaks when the page is published. Weirder again: this does not happen on my site cloned to another hosting server (well to be honest it does, but… pretty rarely, and publishing again fix it!).

Also, If I create a brand new page or post or anything and add a block of this Plugin, it will be broken when published. It’s like anything updated or created with the best WordPress cache plugin will not be working anymore so I can’t touch my site…

Things you may find useful:

* Both the plugin, wordpress and the themes have been updated multiple times in the last 3 months and are now up to date: didn’t fix anything.
* I tried deleting the plugin and reinstalling it again. I tried deactivating other plugins too.
* I couldn’t find anyone having the same issue in the support forums, which tends to make me think that the plugin is quite functional usually.
* I found out that the html classes of plugin elements change everytime I edit&publish, for instance, the title “LATEST NEWS” go from “< div class=”ultp-block-399cad”>” to “< div class=”ultp-block-f9e072″>” while the CSS properties stays at “#ultp-block-39cad { }” . So if I’m not mistaken, it means that the CSS is not updating. But why? What can cause a plugin to update HTML but not CSS all of a sudden? Can there be a server cache for css files? Do you have any other ideas?

Help me