When typing eg. /attribute @s attack_damage modifier add foo 0 and then typing v, add_value is not suggested.
I believe that this issue and MC-312332 are the only extant instances where parts after an underscore are not considered. Other command arguments with underscores, such as /particle and then vi or /scoreboard objectives setdisplay and then a, work correctly.
Can confirm this issue on 26.4 Snapshot 2.
When typing /attribute @s attack_damage modifier add foo 0 v, add_value is not suggested and the argument is shown as invalid command.
As a comparison, typing A in the same position correctly shows the options beginning with add, which was all of them.
This reproduced consistently for me and seems to confirm that autocomplete is only thinking about the part before the underscore.