I suspect that that's par for the course.
The connection in Bern at 18:04 on Sunday is an InterRegio (IR) train. Those are (slightly) slower trains that make more stops and travel shorter distances than InterCity (IC) trains, and they generally have less demand for first-class accommodation, and correspondingly fewer first-class cars. The SBB website also notes that that particular train is usually fairly full in 2nd class between Bern and Romont.
The direct train is an IC and will therefore have more (and perhaps more comfortable) first class seats.