Irregular period. Like this is common!

I am asking for my sister . In 2020 In may month she got her periods on may 7 . In june month it was delayed . So after consulting doctor she had duphaston twice a day for 3 days from 18 to 20 june . She waited upto 30 june . She didnt get period by that time . On july 1st she went to doctor . Doctor got ultrasound done for her . It was all ok . Doctor told it was sectetory phase . Endometrium is thickened . So no need of any medicines . She got period on july 13 . I need to ask why she didnt get withdrawl bleed with duphaston . Was dose less . After that episode now in 2024 again she missed her period and pregnnacy test is negative . Family doctor told her to have meparte 10 mg for 5 days . Is this dose sufficient . Will she get her period . She normally didnt have any irregularity in periods except these 2 episodes . She has 2 year old son . she is 37 years old . Are these irregularies like this is common