I had a similar problem with my T-Mobile LG G2 where I was rooted and accepted an OTA. It broke root and I couldn't get IOroot or Stumproot to work. Went to flash a downgrade and found the phone wouldn't go into download mode. It was NOT bricked. I finally tried playing around with the LG Mobile Support Tool (B2CAppSetup.exe). Of course it detected that my phone already had the latest firmware from T-Mobile, but then I clicked Options>Upgrade Recovery.
It downloaded for a while then I noticed it had put my phone into download mode (it said "Rooted" in red letters at the bottom). Then it flashed the same firmware over and I immediately tested, it entered download mode right away. But I didn't end up needing that as Stumproot was able to brute force it now after the recovery flash.
Anyway, I couldn't find an answer anywhere else. Most results also have a bricked phone, but that wasn't my issue. This solved my problem and maybe it could work for others.