Sharp increase in negative vertical speed in Loiter and RTL modes

Please help me figure out the problem. The copter (frame F330, APM2.8 on vibration isolation, firmware 3.2.1, powered by a current sensor - power module) flew on PIDs by default, it was staggering, but tolerable. Carried out auto-tuning. The swaying has ceased, but now it often tries to spontaneously increase its vertical speed while descending, up to a collision with the ground. This is not a fall, but a stabilized landing at high speed (I’ve seen up to 4.5 m / s on FPV). We fly mainly in Loiter and RTL modes. In these modes, the described phenomenon occurs.
Before autotuning, we flew a lot and there were no such problems. Previously, the GPS module was replaced (Ublox M7N). The old battery ran out and after replacing it, it has become unstable to determine the position. Once I could not give 3D FIX for 30 minutes. Was replaced by a new one (Ublox M8N. As it turned out later - counterfeit with the impossibility of flashing). We flew with the new one without any problems. Autotuning has been performed with a new module.
We tried to roll back PIDs to old ones, reload firmware, load default parameters. No result.
I tried to analyze the logs. Started with DAlt and Alt. When dropped, they match. Ie he “wants” to fall?
Then ThrIn (aka RC3) and ThrOut. At the moment of the fall, ThrOut is at its maximum. And ThrIn becomes maximum with a delay (until we figure out that we need to pull the stick).
ThrOut and AngBst. At the moment of falling AngBst increases and becomes equal to ThrOut or even exceeds it, but with a “-” sign. It looks like this setting is cumulative with ThrOut. If this is the case, then nothing should come out of the regulators at all, which seems to be true.
And the most mysterious pair is Alt and GPS / Vz. It is not clear to me how, when climbing, Vz can be negative and when descending, it can be positive? I thought this was the case. I changed the GPS module back to the old one. This time he gave 3D FIX. But nothing has changed.
In which direction to look is not clear. An internet search yielded nothing