On created UHD BD, eac3to reports only 1 subtitle frame in every subtitle stream. But subtitles are well played. 
Code:
1: Chapters, 29 chapters 2: h265/HEVC, 2160p24 /1.001 (16:9), 10 bits 3: h265/HEVC, 1080p24 /1.001 (16:9), 10 bits - Dolby Vision Enhancement Layer 4: AC3, Czech, 5.1 channels, 640kbps, 48kHz, dialnorm: -27dB, 11ms 5: TrueHD/AC3 (Atmos), English, 7.1 channels, 48kHz (embedded: AC3 EX, 5.1 channels, 640kbps, 48kHz) 6: Subtitle (PGS), Czech 7: Subtitle (PGS), Slovak 8: Subtitle (PGS), English s07 Extracting subtitle track number 7... v02 Extracting video track number 2... s08 Extracting subtitle track number 8... a05 Extracting audio track number 5... a04 Extracting audio track number 4... s06 Extracting subtitle track number 6... Video track 2 contains 206346 frames. Video track 3 contains 206346 frames. Subtitle track 6 contains 1 caption. Subtitle track 7 contains 1 caption. Subtitle track 8 contains 1 caption.