"Matthew" did not know gLuke:
Mk8:15 "Take heed, beware of the leaven of the Pharisees..."
in the missing block reappears in Lk12:1b "Beware of the leaven of the Pharisees, which is hypocrisy."
. Luke's version is very similar of what shows in GMatthew 16:6 & 16:11 "... Take heed and beware of the leaven of the Pharisees ..."
also in gMark missing block (Mk6:47 to Mk8:27a).
Note: "Matthew" knew about the missing block in GMark which he "copied" and expanded (Mt14:24 to 16:13a).
gLuke is without the missing block (except for the aforementioned saying). Therefore "Luke" did not get the leaven saying from gMark or gMatthew, but obtained that saying regardless: from where? Because not from the other Synoptic gospels, from a separate "Q" document. Furthermore, this saying in gLuke (12:1b) is right in front of a series of five other "Q" sayings (12:2-10): that would greatly increase the probability Luke's leaven saying comes from "Q".
Note: "Luke" knew about gMark (without the Markan missing block). See http://historical-jesus.info/appf.html
gMatthew and gMark:
How does that work in practice ? Matthew is aware of Mark's gospel but his audience is not ? So Matthew can pass his expanded plagiarism of Mark off as an original work ? How is Matthew in a community of like-minded Jesus followers but only Matthew knows of (and has access to) Mark's written gospel ?
I don't think gMatthew was presented as plagiarism of gMark. Rather the opposite: gMatthew was told to be the original gospel, later used by gMark for a Gentile audience. Yes gMark was known to gMatthew audience first, but gMatthew would be claimed as lost and then "discovered" later.
Mk16:4 "And when they looked, they saw that the stone was rolled away: for it was very great."
That looks very suspicious: was the tomb opened by disciples in order to carry Jesus' corpse somewhere else?
"Matthew" went in great pain to show it was not so:
Mt27:66 "So they went, and made the sepulchre sure, sealing the stone, and setting a watch.
Mt28:2 "And, behold, there was a great earthquake: for the angel of the Lord descended from heaven, and came and rolled back the stone from the door, and sat upon it."
That is obviously in reaction to gMark:
Mt28:12-15: "And when they were assembled with the elders, and had taken counsel, they gave large money unto the soldiers,
Saying, Say ye, His disciples came by night, and stole him away while we slept.
And if this come to the governor's ears, we will persuade him, and secure you.
So they took the money, and did as they were taught: and this saying is commonly reported among the Jews until this day."