Bitter Cucumbers and cross pollination
I have recently started harvesting cucumbers and have given some away to family and friends. However, some of them have an extremely bitter and unpleasant taste. The fruit and plants look healthy.
The clue here is that you are growing two different varieties of cucumber close together. Indoor varieties are well known to develop bitter flavours if allowed to be pollinated; however outdoor varieties need to be pollinated to make fruit. So the issue is one of cross pollination between the two varieties. The resulting fruits are not always bitter because some flowers will have been self-pollinated.
Next year, plant different varieties well away from each other on your plot.
A word of warning, bitter cucumbers can be poisonous if consumed in large quantities.