When this animation is processed, the system displays 4-color characters from #4 to #7, every pattern is shown for 2 frames.
And where the object number is calculated, it divides the counter by 2 and adds 4. But because it uses the rrca opcode to divide it, when the counter is an odd number, the highest bit becomes 1 which results $84 - $87 values.
This code is located at $345d - of the main program and the problematic rrca is at $3464 of CPU1.

Sprite Controller:
The 4-color objects (4N) are 32Kbit ROM, it doesn't have the 6th bit char A12.
The 7th bit is used for ROM selection of the OBJ data. When the 7th bit of the char number is HIGH and the bit to assign the type of object is HIGH, ~CE of the 4-color OBJ ROM is set HIGH, which means no ROM is selected.
In this case, the sprite controller receives data with two HIGH planes and one LOW plane, means 011 (= 3) is the color of the pixel.
The color table of this flying enemy is at $26 - $29. The color 011 (=3) at $26-28 is transparent and $29 is red, this shows the red block.

I guess the programmer could have forgotten to add "and 07h" after rrca at $3464 of CPU1.