On QFX5120 series switch, warning message below might be seen when exceeded 2049 vlans on trunk LAG interface.

warning: Too many VLAN-IDs on untagged interface

This is a software bug as QFX5120 has supported more than 2049 vlans on the same trunk LAG interface.

QFX5120 would generate warning message below when configured more than 2049 vlans on trunk LAG interface.

user@Switch# commit check
[edit vlans v1]
warning:  Too many VLAN-IDs on untagged interface
configuration check succeeds

But if they used vlan numbers with range string, the message is not seen even if exceeded 2049. And the issue is seen with LAG interface only, not seen with normal interface.
Here is the sample config to see the issue.

user@Switch# show interfaces ae0 | display set
set interfaces ae0 aggregated-ether-options lacp active
set interfaces ae0 aggregated-ether-options lacp periodic slow
set interfaces ae0 unit 0 family ethernet-switching interface-mode trunk
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan1
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan2
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan3
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan4
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan5
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan6
~snip~
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan2043
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan2044
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan2045
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan2046
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan2047
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan2048
set interfaces ae0 unit 0 family ethernet-switching vlan members vlan2049

{master:0}[edit]
user@Switch# show interfaces ae0 | display set | match members | count
Count: 2049 lines

This is a software bug which reported by PR1855085. This is a warning message and there is no functional impact.
https://prsearch.juniper.net/problemreport/PR1855085

You can use range command below for a workaround.

set interfaces ae0 unit 0 family ethernet-switching vlan members 1-4000