losing the back under front braking...sometimes?

[Val]

The lighter the bike the more significant the rider’s weight becomes… the centre of gravity may well have moved upward as a result.
What is the state of tyres?
Is the suspension set up for you?
If the front spring rate or preload is a bit low for your weight or if the fork oil is too light then you’ll get more dive… more dive transfers more weight to the front and takes it off the rear.

True I am heavy 109kg. Tyres look great. The suspension is set up for me, but there are not much options here, means it is jacked up almost on max. When I brake there is not big dive.