this is one of the reasons why I never recommend duct cleaning. esp flex.

whoever replaces the flex, make sure that the connections are sealed with
either paint on mastic or mastic tape. from hvac supply now lowes.
for mastic tape, I only use hardcast brand 1402 tape.

ducts should be straight tight runs with gentle curves. no more than
1" sag in 4' section. 3" duct straps cause less restriction than smaller widths.

best of luck.