Ublox M8U GPS gives "unhealthy gps signal" even with 3d+dgps lock

Hello all,
I have changed an ublox M8N gps in a rover (ardurover 4.1.1 official on a pixhawk 1) by a sparkfun M8U (with dead reckoning). Now, I get a “unhealthy gps signal” in mission planner even with gps status = 3 or 4 , and a solid and stable position, with HDOP = 6 and 21 satellites in view.
I cannot figure out what is happening and what could be the problem. Is it a ublox model not supported? in the message window I can see:

12/10/2021 15:38:57 : u-blox 1 HW: 00080000 SW: EXT CORE 3.01 (16559b)
12/10/2021 15:38:55 : GPS: u-blox 1 saving config

Any help will be appreciated.
Thank you in advance and regards,

This problem arised in mine too in Arducopter since i updated the Here 3 GPS and the problem was gone.Do trying updating the firmware if possible.

thankyou! will try a firmware update.

There has been some recent discussion on this in other threads. Ex:

The issue is possibly the uBlox GPS running at 203400 baud on a UART without DMA. You can try setting GPS_DRV_OPTIONS to 1 to force uBlox to 115200 baud: