I played a little with sample rates, sine wave frequency and FFT plots and eventually figured it out. You can find some details in this discussion What exactly LimeSuite Sink (TX) does with IQ data? Apparently LimeSDR doesn’t work as a signal generator particularly well on frequencies < 90 MHz. Especially it doesn’t like frequencies that don’t divide by 30 MHz, I guess it has something to do with internal clock: Rakon RPT7050A @30.72MHz.
TL;DR it’s a bit tricky, but you actually can use LimeSDR as a signal generator (I got sine waves up to 500Mhz, my oscilloscope can’t show larger frequencies) if you keep in mind stuff like how aliasing depends on sample rate and are willing to tune the gain manually (it changes with frequency and is much higher if frequency % 30Mhz == 0).