I cannot produce the buckskin horse with no markings using spawn eggs. I have attempted around 1,000 times in the latest 1.14.1 release to no avail. From my understanding, there are 7 colors and 5 different marking patterns for a total of 35 possible outcomes. If I have truly tried 1,000 spawns, that does mean I only had a 28.6% chance of spawning one, but it has been a few hours of attempts and I feel this is statistically significant enough to report as a possible bug, especially given the fact that I have spawned at least 1 of all the other 34 horse combinations.