Afghanistan is not a full fledged test playing team while Bangladesh is a regular test playing country. So performing good against an associate team does not prove the fact that Bangladesh is very good at home conditions. Bangladesh were recently whitewashed by Pak in Test and T20Is at there home grounds. Afghanistan is an underdog in this series.
With the popularity of short formats now it's very hard for few countries to have better result in this test format because it needs some good patience and better technique which are time-consuming things and new breed of cricketers love to enjoy ODI and t20, so test failure is increasing for new countries very strongly.
Bangladesh is not giving time in domestic set up which is also very important for this and Afghanistan is not able to play this at home, so they are just enjoying short format but still have some good players for giving good response in test format, but I am feeling they will be not good enough in near future until they also have some strong domestic set up which help their players for having some good experience in this format for better success ratio. Right now, we have strong domestic set up in very few countries, so they are enjoying good in all formats.
Bangladesh has always had problems in test cricket. The best form of cricket in which Bangladesh plays really well in the one-day international.
In T20 Bangladesh is below average in my opinion, in test cricket to be honest Bangladesh absolutely sucks. one of the big reasons for this is because Bangladesh doesn't have different lineups for different formats.
Mominul Haque is a great cricketer for Bangladesh and he made a great name for himself when he started his career in test cricket but then the over-eager selectors selected him for the one-day and T-20 matches and he did not do very well in those. So his form in test cricket also dropped.
The domestic setup is well in very few countries. The domestic setup in Bangladesh is not good at all and a lot of match-fixing by the umpires and some shady business going on in the domestic cricket of Bangladesh, you know. I think no one remembers the bowler in Bangladesh domestic cricket who
gave 92 runs in 4 balls. Yeah, that also happened. Now that bowler can be found selling burgers in the streets of the City, kidding,