In the first innings, the Bangladesh team made 247 runs and we all thought how poorly they batted in that innings. To everyone's surprise, in the 2nd innings, they batted even worse and were all out at 147. Yes, they lost the match but there are some series concerns with the Bangladesh batting.
There are some good players in the team like Litton Das , Najmul Hossain Shanto and Mushfiqur Rahim but none of them seems to bat for long. The Bangladesh top to middle order, everyone had a start but then when they got settled and it seemed they could play a big responsible innings, they kept getting out.
You have accurately pointed out Bangladesh main batting problem despite having good players like Litton Das and Najmul Hossain Shanto and Mushfiqur Rahim they consistently fail to turn good starts into big scores mostly getting out just when they look settled. This repeated pattern of batsmen throwing away their wickets after getting comfortable is really hurting team making it hard to score enough runs and leading to losses.
The only failure of Bangladesh batsmen here is that they cannot get set or even if they are set, they cannot bat with determination for a long time. If it is seen that they have set, then they play such a wrong shot and get out. Until Bangladesh can refrain from such wrong decisions or can come out, Bangladesh will not be able to improve in Test cricket. Whereas players from other countries can bat with patience in the face of adversity, Bangladeshi players have the skills to perform well, but due to a few wrong decisions, Bangladeshi players perform poorly and get out in every match.