I’m not entirely sure what the bug is, but there is an issue when researching the Improved Lightweight Beams Tech. Depending on the order the Tech is researched in conjunction with Improved Military Academy, the train time in a siege workshop ranges from 40 Seconds to 1 min 30 seconds. I believe the Improved version is for whatever reason applying the default field construction time to the workshop instead of keeping it the same. I did not notice it on other Civs so I believe its only due to the improved version.