It's not necessarily the completion percentage but the TD rates:
2018 - 23.7%
2019 - 20.8%
2020 -19.2%
2021 - 19.3%
2022 - 19.3%
Those are pretty bad, especially when you tie in the interceptions in the red zone. Hopefully, Sneaky Pete can scheme that clean because just for grins and giggles, I checked 2021 stats, based on volume highest number of attempts:
Siemian - 17/35 (48.8%), 123 yards, 10TDs/0INTs, 28.5% TD Rate, 97.38 passer rating (Siemian also had 1 rushing TD in the RZ)
Winston - 16/21 (76.2%), 147 yards, 11TDs/0INTs, 52.3% TD Rate, 134.33 passer rating (Winston also had 1 rushing TD in the RZ)
Taysom - 7/13 (53.9%) 46 yards. 2TD/0INTs, 15.3% TD Rate, 101.28 passer rating (Taysom also had 4 rushing TDs in the RZ)
Siemian threw TDs at a higher rate than Dalton did this past season.