This is all interesting. Based on your research, you have determined that the original 99 corridor was Chester Ave to 19th streeet to Union. Essentially, forcing a turn in the middle of Downtown Bakersfield.
Chester goes right through the heart of Bakersfield, and Union is a good ways to the east and makes a decent first-level bypass, which is what eventually happened once the Golden State Ave diagonal route came to be. Not only does that bypass avoid the heart of downtown, you also avoid two 90 degree turns, in favor of more gradual turns along the diagonal.
When looking at a map of today's Bakersfield, one sees that Chester Ave actually goes diagonally to the southeast to meet Union Ave near the municipal airport. This roadway seems to have come about far later than any of the old US 99 alignments discussed in your article. Does anyone have any idea as to what could have promped this connection? A desire to force a business route directly through Chester that will meet the original 99 well south of town? A definite curiosity.
While not knowing the true reason behind the historic society saying that 18th (rather than 19th) was the crosstown connector between Chester and Union, perhaps it has something to do with the fact that 18th is two-way and 19th is one-way. Obviously, those traffic controls came about long after 99 left 19th, but if one wanted to retrace the old road in some way, perhaps it is easier to do so on a two-way road. Or perhaps, as the linked article did mention, the city preferred long distance traffic on 18th to avoid the trolley that was on 19th.
We had a great thread here a couple years ago discussing how frequently 101's alignment through central Los Angeles was changed: 7th street, 2nd street, and Macy street were all at different times the 101's main pre-freeway routing. Many of the routes changed from year to year. Also, the north-south connectors on the west side of town to reach Sunset and the east side of town to reach Beverly, Whittier, or Telegraph also changed pretty frequently.
See:
https://www.aaroads.com/forum/index.php?topic=23727.msg2425478#msg2425478I could imagine similar considerations affecting Bakersfield. Until the Golden state ave bypass was constructed, the city authorities (with or without Caltrans permission) could have decided that 18th was better than 19th and changed the on the road signage, even though the official state maps all had 19th.