Starting about May 2, 2021, when I read messages in a browser looking at the Messages List heading, the links are no longer being marked as being read as I read through one post after another.
In other words, the links still show blue after I have read through several. The problem appears to be that "Next" arrow points to a "sparse" link rather than the full link as seen in the Messages list view.
eg) List view shows: https://beta.groups.io/g/main/message/29318?p=,,,50,0,0,0::created,0,,1,2,0,29318
<,> links use: https://beta.groups.io/g/main/message/29318
This has been working for several years [ reference: https://beta.groups.io/g/main/topic/943197#7386 ] and now appears to be broken.
This was also reported on a help group, https://groups.io/g/Group_Help/message/2579. It seems to be using the next/previous arrows that creates the problem. This may be related to https://beta.groups.io/g/main/topic/82515813