Lacking full knee extension creates a functional leg length discrepancy. This will cause a person to compensate somewhere else (ankle, hip, back, etc.).
According to one study:
“On average, 1 in 3 patients may present with loss of extension of at least 3 degrees at 12-month follow-up, decreasing to 1 in 4 at 2 years.”
There are many ways to get knee extension back. The first step is recognizing that you have this deficit.
https://pubmed.ncbi.nlm.nih.gov/36705690/