Learn Portuguese (Portugal): Temos testado a dinâmica recentemente. Ele tem-na achado jogável. - We have been testing the dynamic recently. He has been finding it playable.

In compound tenses made of ter plus a past participle, object pronouns always attach to the auxiliary verb (here, tem). You can never attach a clitic pronoun to a past participle in Portuguese, so tem achado-a would be grammatically incorrect.

Why is the pronoun -na instead of just -a in tem-na?

Because tem ends in a nasal sound. Whenever a verb form ends in a nasal sound (like -m, -ão, or -õe), the direct object pronouns o, a, os, and as gain an n to make them easier to pronounce. Here, the pronoun refers back to the feminine noun dinâmica, so a becomes -na.

Both sentences translate to "have been [verb]-ing". Is that always how the Portuguese present perfect translates?

Mostly, yes! Unlike English, the Portuguese compound present perfect specifically describes an action that started in the past and is still repeating or continuing into the present. Because of this continuous nature, temos testado translates much better to "we have been testing" than "we have tested".

Could we use the verb encontrar instead of achar to say he found it playable?