It’s not Chrome, or not just Chrome at least: I’ve been seeing the same issue with Firefox for Android. It’s a relatively recent change, too. Last two or three months, I’d guess? (I didn’t exactly keep notes when I first noticed it.) Previously, Firefox would always show the thread title, and would hide the thread tags more aggressively.
And yeah, it seems to be tied to (or caused by) the number of tags a thread has. Three or fewer, including subforum icons? All in one row, title fully visible. As it used to be. Beyond that? The extras stack vertically at the end, gradually pushing the title upward out of sight.
At this point, I’m personally blaming Discourse.
I’ll admit, I don’t see a reference to which mobile browser @princeraoden is actually using? But I’ve tested with both Firefox for Android and Samsung Internet Browser. Both experience the issue. Wikipedia claims Firefox for Android uses a rendering engine called “Gecko,” while Samsung Internet Browser uses “Blink.” And if OP is using an Apple phone, then their browser is using “WebKit” for its rendering engine, as that’s apparently mandatory for all web browsers running on iOS. Assuming Wikipedia is correct and up to date on the topic, then I wouldn’t expect the issue to be on the browser end.
For reference, the two threads featured in OP’s screenshots are The Wight King (which currently has one tag, plus the Hosted Games and Works In Progress subforums) and A Mage Reborn (which currently has four tags, plus the same two subforums).
For further comparison (and possibly relevant if drafting a complaint to “The Powers That Be” in charge of Discourse), none of the desktop web browsers I’ve tested have pushed tags into the title area the way the mobile browsers seem to. For example:
Notice how the mouseover popup contains the thread title, the two subforums, and the overall forum… but not any of the four tags?
All that aside, I’m not sure how much this matters? Sure, it’s a stupid change, and it makes the site slightly more annoying to use on mobile. But at the same time…
As you already pointed out, tapping the post counter reveals the full title. Not only the full title, but any and all tags, too! Which is, ultimately, why I didn’t bring this up myself when I first noticed it. It’s stupid, it’s annoying, but it seems easy to work around.