How to fix Cmd-Alt-P-R booting into recovery?

Could be you aren't holding all the keys down simultaneously or long enough. Here is a way from the Terminal's command line: sudo nvram boot-args=”-p -r” or you can try instead, nvram -c. Do not know if either still works. Then you might try this:

Resetting the NVRAM on Late 2016 MacBook Pros and Later

MacBook Pro models introduced in late 2016 have a slightly different process for resetting the NVRAM to its default values. While you still hold down the usual four keys, you no longer have to wait for a second reboot or listen carefully to the startup chimes.

1. Shut down your Mac.
2. Turn your Mac on.
3. Immediately press and hold the command+option+P+R keys.
4. Continue to hold the command+option+P+R keys for a minimum of 20 seconds; longer is fine but not necessary.
5. After 20 seconds, you can release the keys.
6. Your Mac will continue the startup process.

I've never used a firmware password. I suggest you might try disabling that for now. I'm guessing that the computer cannot be restarted without that password, hence, it is reverting to the Recovery HD. Or your disk drive has failed.

I've been thru the same thing. Don't know why it doesn't always work. Could be because of using the BlueTooth keyboard. I've never had a problem with a wired keyboard.

Oh, be sure you hold down the keys until the chime has been heard. No problem holding it more.