After a long pause when all my new findings came from data mining, I eventually found a new variable star on my own digital images.
The star is registered as PMAK V136 in the International Variable Star Index (VSX): https://www.aavso.org/vsx/index.php?view=detail.top&oid=2388114
It is a Gamma Doradus-type star (GDOR) whose variability is caused by asteroseismic oscillations. In most cases, the variability range of such stars does not exceed 0.1 V. My observations (clear filter) show the amplitude of 0.1 mag. However, there is evidence that GDOR stars may show a variability range well above the 0.1 limit (see, for example, 2020MNRAS.499.3976P). Such High Amplitude GDOR stars (HAGDOR) oscillate on several close frequencies; beating between those frequencies may give higher amplitudes. It seems that PMAK V136 is such a HAGDOR star: ZTV g variability range is 0.25 mag.
I thank Sebastian Otero for his help in the classification, advice, and tips.