I think I have worked out how Lv. 1 sandwiches work, and I have a strong suspicion that the flavor-based power determination system described below may also apply to the second power of sandwiches made with one Herba Mystica or the third power of sandwiches made with two HM.

With Lv. 1 sandwiches, the top power seems always to depend on the strongest two flavors in the sandwich. If the two strongest flavors are sweet and sour (in either order), the result will be catching power. If the two strongest are sweet and hot (in either order), the result will be raid power. If the two strongest are salty and bitter (in either order), the result will be exp. point power. For the remaining combinations of flavors, only the strongest flavor matters. Sweet (not paired with sour or hot) will yield egg power. Salty (not paired with bitter) will yield encounter. Sour (not paired with sweet) will yield teensy. Bitter (not paired with salty) will yield item drop power. Hot (not paired with sweet) will yield humungo power. Ties seem to be broken with the order sweet>salty>sour>bitter>hot (the order in which they appear in datamines), but I've not fully tested that. The resulting flavor of the sandwich appears to be the sum of the flavor in all the ingredients. Any ingredient which is placed multiple times (like strawberries) has its flavor values multiplied by how many slices of the ingredient you place (so for example, strawberry actually adds 12 sweet and 12 sour, not 4 sweet and 4 sour as suggested by the datamine).

The other two powers chosen are the remaining powers with the highest score. As with the flavors, the powers' scores are simple sums (taking into account the multiplying effect for ingredients added in multiple slices mentioned above). Ties among the powers also seem to be broken in the order in which they appear in the datamine (egg>catching>exp.>item drop>raid>humungo>teensy>encounter). Title and Sparkling appear to be special in this respect since they are determined by Herba Mystica added, and in general these two powers seem to follow their own rules and take precedence also over the power chosen by the flavor of the sandwich.

The types associated with each power are taken from the three highest scoring types. Ties are broken again by the order in which the types appear in the datamine (normal>fighting>flying>poison, etc). The scores of the types seem to be calculated just like the scores for the powers by simple addition. The types seem to be assigned in a quirky fashion. The highest scoring type goes with the first power, the second-highest scoring type goes with the third power, and the third-highest scoring type goes with the second power.

This system seems to explain both the resulting powers and types for sandwiches made in creative mode as well as sandwiches made from recipes. What I have not figured out, however, is what determines the level of each power. For example, if we take the great pickle sandwich (pickle, watercress, olive oil), and replace pickle with yellow bell pepper, we get a sandwich with a similar flavor profile, similar power values, and similar type values (just with electric boosted instead of fighting), but the item drop power of the resulting sandwich is Lv. 1, not Lv. 2. This part is still all rather baffling.