Hopefully the following can give some insight:
In the config.json settings, I checked before logging into the game, and the PlayerResourcesSet (which I presume is the setting that this is dependent on) was the following:
Which means that my mana/hearts was set on the "bars" mode. However, upon loading up the game and logging onto the game, and specifically into a character/world set on Journey Mode, it was reverted back to fancy, and the setting changed to the following:
As such, I changed the setting back to the "bars" setting. This made the PlayerResourcesSet go back to the HorizontalBars setting in config.json. Upon further testing, logging in and out of the world (without closing the game itself) kept the PlayerResourcesSet constant, so that the bars were still present upon loading up the same world. The same happens after closing the game out completely. It is also the same when I completely close steam and then launch Terraria.
So, I'm thinking - if it doesn't occur under any of these circumstances, what did lead to it occurring? A few thoughts come to mind - namely, this was the first time I had booted up Terraria for the day (or, in fact, in several days), and as such is the first time the game has been loaded since 22.214.171.124 has been released, somewhat reinforcing Leinfors' assertion that it's due to hotfixes resetting the settings (though that still begs the question to me of why it changed upon startup of the game itself, or perhaps in the login - I have Terraria set to auto-update, so the update shouldn't have been enforced as soon as the game loaded; additionally, I seem to remember that the same issue occurred without an update occurring, though it's impossible to double-check that now - I definitely played several times in between the release of 126.96.36.199 and 188.8.131.52, assuming the Wiki's release dates are correct, and I seem to remember the same issue occurring then).
With all this in mind, I'm going to try and restart my computer to see if the same issue happens - I'll edit the post once this has been done to say whether it actually affected it. If not, I have no clue, but hopefully, this information can help the development team figure out the source of the issue.
Cheers! Be back within an hour with an update on how restarting my computer worked.