XPS 13 (7390) - AC adapter and BIOS upgrade issues

I have a XPS 13 7390 laptop with Ubuntu Linux pre-installed, bought in November 2019.

Today when I turned it on, I noticed the battery icon indicated that AC power was unplugged and the battery was draining. The thing is, the AC adapter was in fact plugged in, and its indicator light was turned on. Resetting the machine and trying other wall plugs didn't make any difference. I didn't have any peripherals plugged to the laptop from the beginning, so that too was out of the question. Resetting to BIOS, I found that the AC adapter was listed as "not installed" (or something to that effect), showing the problem ran deeper than the OS.

After some forum searching I decided to upgrade the BIOS from version 1.3.1 to 1.5.1, which did solve the charging problem: the AC adapter was now shown as plugged in the BIOS status screen, and the battery started recharging. But now Linux wouldn't boot, and even some BIOS functions such as system diagnostics would hang, forcing me to do a hard reset.

After trying some system fixing options without success, I decided to rollback the BIOS to 1.3.1, which solved both the OS booting and battery charging problem.

While it's nice that everything worked out in the end, I am left with a few lingering questions:

1. Why did the AC adapter stop being recognized all of a sudden, for no apparent reason?
2. Why did upgrading the BIOS to version 1.5.1 (an operation listed as "urgent" in the driver database) leave me with a half-borked system?
3. Why did rolling the BIOS back to 1.3.1 fix the AC adapter recognition problem?

Honestly I'm half-resigned I'll never get any answers, but I'm recording the questions here for future reference if nothing else.