Hi,
Although, only momentary loss would be expected after failure of a member LACP link, one of the contributing factors for further loss in this scenario is ECMP and default route to 20.20.20.2.
When irb.20 is deleted in L3_2, I assume irb.20 would stil be up on L3_1 and the default route to 20.20.20.2 would still be present in L3_1's routing-table. Any packets ECMP routed to 20.20.20.2 would thus be lost. You could possible use BFD with static routing to monitor the next-hop which will cause the default route to 20.20.20.2 to be withdrawn on L3_1 when irb.20 is deleted on L3_2.
Cheers,
Ashvin