continuous ping fails when backup default route becomes active

hello. wonder if anyone can help me.

i have secured my network with 2 routers (vrrp for hardware redundancy), each one of them connected to it's own wan access (R1main - fiber and R2backup - lte/4G).

to test the performance of this solutions i made a continuous ping to a responsive domain name, using both a laptop with win10 and another one with ubuntu.

when i test the hardware redundancy (vrrp) the continuous ping keeps going, not even droping a single ping, wether i poweroff the main router or put it back on.

but when i disconnect the fiber and my backup default route (to the R2backup- 4G) becomes active, i do continue to have internet access on both laptops, but the continuous ping stops responding on both laptops. if i connect the fiber again on the windows laptop it starts resuming the pings, but the ubuntu laptop continues to fail the pings, and i have internet access. why is that behaviour with the pings when it changes to the backup default route and resume the main route????

the routes seem to be doing the job, as i checked with a tracert/traceroute to a responsive internet domain.