Maybe dynamic is a nullable field, but is not a nomal condition for other dynamics in the system.

/interface pppoe-server
add name=pppoe-in1 service="" user=teste

NOT WORK
print where dynamic=no
NOT WORK
remove [find dynamic=no]

WORK
print where !dynamic
NOT WORK
remove [find !dynamic]

WORK
print where dynamic!=yes
WORK
remove [find dynamic!=yes]