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I would give the plant more time; especially since the branches sound alive and viable. You may watch at the base of the perennial. Often time it will grow from the root, not the remaining spent branches.

https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-care.htm

Butterfly bush is late to emerge so hang in there. In MA, they will die way back in winter so cut them down to 9-12 inches late fall or early spring.

Butterfly bush dies back almost to the ground in winter. Just how far back it dies depends on where you live. In Michigan, people cut their butterfly bushes back to 9-12 inches in late fall. So what you are seeing is normal. New growth starts at the base but may inch up into what remains of last year's branches.

These articles will help you pinpoint the issue.
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-diseases.htm
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/brown-leaf-spots-on-buddleia.htm
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-care.htm

I would selectively prune then cut back hard in the late fall

If you cut it down to 12" in late fall, piling chopped or shredded leaves over the top should be all you need. It will die back every winter in zone 5b but is reliably hardy. Use the lawn mower to chop/shred some leaves to mulch the crown. Covering with a terra cotta pot makes sense except it will be prone to cracking as it absorbs moisture then suffers freezing temps. Freeze and thaw cycles have ruined many clay pots.