I frequently find that my data service is completely broken even when I have full 5G bars. Inflating by one is lame but doesn't explain this behavior. Is this a T-Mobile thing or is it widespread these days? I don't remember it happening so much 3+ years ago.

Signal strength is a measure of how proximate you are to the tower in terms of radio connectivity, but it says nothing about whether or not the tower will respond to you in a timely fashion, the tower backhaul capacity, etc. Usually this happens because you have a great connection to the tower in theory, but in-practice you can't get meaningful bandwidth and everything appears broken. This is really common at sporting events and other large crowd gatherings, which is also why a lot of the promise of 5G was that increased work with OFDMA in trying to service more customers in the same physical space adequately.

It's probably a reasonable pitch to say that phones should instead display something closer to "meaningful available bandwidth" crossed with strength, because a strong signal doesn't mean a good connection.

Maybe related to 5G? There are a couple spots near me (in particular, a somewhat crowded open mall) where I have solid bars but zero connectivity. Dropping to 4G works in most cases.