Hi @Roumpf, do you have some screenshots of this issue and/or URLs to example pages where the problem is happening? Thanks!

Also had this issue. As a temporary workaround, go to the MediaWiki:Timeless.css page on your wiki and add:

table {
  border-collapse: separate;
}

To reproduce this issue, simply try creating a table with rounded borders on a wiki:

{| style="border: 2px solid red; border-radius: 5px;"
! Header !! Header
|-
| Cell || Cell
|}

The expected result is rounded corners. The actual result is square corners. This works fine on non-Timeless skins because Timeless is the only one that has:

table {
  border-collapse: collapse;
}

If there isn't a valid reason for Timeless to have that property, please remove it.

The reason for the border collapse was to get 1px borders between the cells instead of 2px, but evidently the default wikitable styles don't need that, which is interesting...

Change 532285 had a related patch set uploaded (by Isarra; owner: Isarra):
[mediawiki/skins/Timeless@master] Kill weird table normalisation

https://gerrit.wikimedia.org/r/532285

Change 532285 merged by jenkins-bot:
[mediawiki/skins/Timeless@master] Kill weird table normalisation

https://gerrit.wikimedia.org/r/532285