I think this is actually a useful design feature - if the thread didn’t move and it was up in the message chain and I wasn’t tagged in a message then I would miss it, but as I want to still read everything in the channel, the fact it comes to the bottom of the channel means I see it.
Apps like Slack solve this by having a dedicated “Threads” channel where you can see new messages in threads you’ve participated in. There’s also the concept of an “Activity” view where you can see recent activity in a channel, if I recall correctly. There’s no need to re-arrange message order or rewrite history to draw attention to new messages. Worst case, just insert a message visible only to you with a link to the thread when updates occur.
