ffmpeg scaler options not working
Use the print_info flag to see what the scaler is doing, example from an actual conversion:

-filter:v "scale=w=1280:h=-2:flags=print_info+bicubic"


Part of the relevant output:

[Parsed_scale_0 @ 000001d8187fef00] w:1280 h:-2 flags:'print_info+bicubic' interl:0
[swscaler @ 000001d819bc7fc0] bicubic scaler, from yuv420p to yuv420p using MMXEXT
[swscaler @ 000001d819bc7fc0] 1920x804 -&gt; 1280x536
Parsed_scale_0 @ 000001d8187fef00] w:1920 h:804 fmt:yuv420p sar:1/1 -&gt; w:1280 h:536 fmt:yuv420p sar:1/1 flags:0x1004


Edit:  I also always use "-loglevel verbose", don't know if that changes the level of detail the scaler filter outputs.