Summary: Deepslates Rotation for Placement and Generation in Bedrock seems to be calculated randomly by a probability to connect to a certain block around it except this effect is applied to generation because it connects to air and in the world generation any thing thats not a block is air meaning that you would get different results if probability of the rotation being calculated mathmatically was with equal probabilities of the rotation for each side of the top texture because its connecting to air and surrounding blocks. This Bug can be seen when a player places deepslate on a block with or without blocks surrounding it the placement is random due to the fact that the deepslate identifies air as a block that it can connect to when placed or generated.

Methods to Reproduce:

Method 1: Create a Behavior Pack
Step 1: Make a Behavior Pack that adds a ore feature.
Step 2: Define the block that supposed to replace stone as deepslate in a ore feature.
Step 3: Create the feature rule area to specify the regions the deepslate is supposed to generate.
Step 4: Load the World and Find Deepslate in Specified Regions that Deepslate can spawn.
Step 5: Repeat.

Method 2: Place Deepslate
Step 1: Load a World and Find Deepslate.
Step 2: If your in Survival Mode mine it with Silk Touch a Pickaxe If your in Creative get the Deepslate Block out of the Creative Inventory.
Step 3: Place the Block on top of a block surrounded with blocks or without.
Step 4: Analyze the Results.
Step 5: Repeat

Method 3: Find a Deepslate Blob at Y 0 -16
Step 1: Load a World.
Step 2: Get the Materials to Dig Down to Y 0-16.
Step 3: Find a Deepslate Blob by Digging a Tunnel in One Direction.
Step 4: Repeat.

Observed Result: The Deepslate seems to be randomized in its generation due to the fact it can associate its texture rotation with air or other blocks ignoring the default placement like Java's thats bi-directional instead of multi directional. When you analyze the pattern you can clearly see that both scenarios get the same result for Method 2. Furthermore when a player does Method 1 the result can be seen here as well.

Expected Result: Deepslates Texture Rotation Shouldn't ignore rotation rules when its placed on a block regardless if there's air or blocks around it to make the rotation bi-directional or align with Java's Deepslate Rotation so that the deepslate can only connect to deepslate in the directions that Java's deepslate can be generated or placed to be.

Another Possible Explanation:
The Deepslate In Java from testing it in a superflat world seems to only generate in rows or columns it seems highly likely if none of those reasons above explain this its because in Bedrock the deepslate checks its placement by checking its alignment in rows and columns instead of Java as seen in the photo below seems to only check one for deepslate specifically for its top texture rotation meaning its not able to connect in the code to surrounding deepslate unless its in that row or column meaning it will always have the proper top texture rotation. Finally Its also possible this bug is linked to the block connecting to air and other blocks and this as well meaning it will never in the current top texture rotation checking or code have the correct rotation to match Java's 100% the time in bedrock.

Solution: Make the Bedrock Deepslate Top Texture rotation be defined by either a row or column of deepslate and prevent deepslate from recognizing blocks or air besides that its being placed on a block outside of that defined region/area so the texture rotation is bi directional to match Java.