Hi,
According to your description, my understanding is that the not unique lookup column display only 1 row data after filtering.
I have create a list named List03_test, and create a lookup column named lookup to get information from other list with Title column. I set the lookup column not unique value. Then I filter the lookup column and it displayed all the items with the filtered
value, shown as figure below.

How did you filter the lookup column?
Feel freely to reply if I misunderstand what you mean.
Thanks,
Dean Wang
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