I’m experiencing this same issue with BART transformer and I created a Stackoverflow post about the issue: https://stackoverflow.com/questions/66996270/limiting-bart-huggingface-model-to-complete-sentences-of-maximum-length 10
Here are some of the output summaries with truncated sentences:

EX1: The opacity at the left lung base appears stable from prior exam. There is elevation of the left hemidi
EX 2: There is normal mineralization and alignment. No fracture or osseous lesion is identified. The ankle mort

Were you able to find a solution to this problem you encountered @hf324?