Is it permissible to keep prohibited bank shares until the capital is recovered after repentance, or must they be sold immediately, even at a loss?

Whoever bought forbidden shares and then repented must dispose of them by taking back the principal and getting rid of the profits by allocating them to charitable causes.

You must dispose of the remaining shares in the usurious bank, as one of the conditions of repentance is to abandon the sin. You are only entitled to what remains of your principal after the loss.

Allah the Almighty said: (Those who consume interest will not stand [on the Day of Resurrection] except as one stands who is being beaten by Satan into insanity. That is because they say, "Trade is only like interest." While Allah has permitted trade and forbidden interest. So whoever has received an admonition from his Lord and desists may have what is past, and his affair rests with Allah. But whoever returns [to dealing in interest] – those are the companions of the Fire; they will abide eternally therein. Allah destroys interest and gives increase for charities. And Allah does not like every sinful disbeliever).

What a person took from usury before knowing [it was forbidden] is permissible for him, provided he repents and ceases [the act]. However, if he repents from usury before taking possession of it, it must be relinquished.

Your remaining [in the transaction] to compensate for the loss is a continuation of the forbidden usurious dealing or an entry into new forbidden transactions, which is impermissible.