This question is complicated and since we don't have access to reading the actual language in the trust(s), it is hard to provide a completely accurate answer.
In these situations, a bypass trust is really two trusts that are set up upon the death of a spouse; there is the marital trust and then the "bypass trust" or it could be called a credit shelter trust (these are the A and B trusts). This bypass trust, or B trust, is an irrevocable trust. Depending on how this is set up, it may or may not be a grantor trust while your mom was living. Regardless, if your Dad owned the LLC interest, then at this point in time, that interest would be stepped up. It would then be adjusted annually for the applicable K-1 line items.
Your interest when that LLC interest is distributed out of the bypass trust, will be the tax basis at that point in time; which should have already included the original step-up.
Keep in mind, the change to having to reflect tax capital account reporting for 2020 partnership and LLC's taxed as a partnership will not always be accurate. The individuals preparing these 1065's don't always have all the information available to accurately determine the beginning balance.
Also keep in mind, that this figure on the 1065 K-1 item L, is only a tax capital account number. It is not your tax basis.
You may want to sit down with a tax professional to make sure you know your outside tax basis beginning point.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
