I’m sure you know this, but it is simply cell availability. The Model Y didn’t exist in 2018/2019. Tesla didn’t sell a lot of cars back then either. I don’t know if it’s “going backwards” as much as managing their supply chain.
As I stated previously, Tesla miscalculated sales of the Model Y vs the Model 3. Everyone knows Juniper is coming, even if not til 2025, so people are holding off on buying the Model Y if they can - unless given a sweet deal on one now like Tesla has been trying to do.
So Tesla pushed all their Nevada cells into the Model Y (except M3P which needs them) and were left short handed for the M3LR. Now they are sitting on tons of Model Y inventory and a lower future sales forecast, so cells are available for the M3LR now. Couple that with people want refreshed models and Tesla was basically forced into going back to the Panasonic cells for the M3LR, which I believe they wouldn’t have changed in the first place had enough cells been available to begin with.
The new Panasonic plant in Kansas is coming online next year, which will solve the USA cell issues for the foreseeable future one would think.