As pool is using PPLNS system for payouts, the 17xxx ports will only switch to the most profitable when a block is found on the currently mined one. This is to prevent users losing their shares submitted (losing credits due to early switching) and it is considered to be a smarter and better way of mining.
Maybe MPH has a plan to add coins in the future, maybe he just doesn't want to in order to save on the storage bill or keep the server loads low, who knows? We have an almost endless list of good coins waiting to be added for months, some of them are requested over 6 months ago. You can request them here but don't expect them to be added any time soon.
The pool indeed has a very limited set of coins for each algo. I guess, we either try to manage it here or find another pool or set of pools. After many years of being a vocal and loyal user of MPH, I had to realise that this pool is stalling out and despite my kind warnings to MPH, this pool does not want to keep its relevance in the game. I, for one, am switching between a few pools and this one now depending on profitability. It is not ideal, requires extra manual work but at least the payouts are much better. Sorry MPH, I am not going to lie, you will slowly lose your users. 90% of my hashrate is already somewhere else in average.
miners will lose credit when they return from mining another coin - first rounds will be paid less, PPLNS on multiport is DONE WRONG, it should sum shares on all equal algos, not like it is now on individual basis.
I agree, when coins mined in a multipool fashion on the 17xxx ports, shares should be counted in continuity (and therefore carried over if applicable) when port switches to another coin of the same algorithm. Using the single pool/coin ports (20xxx) is a different story.