Your card has problems with 640 byte writes.
When a file is created, random writes are required to allocate clusters and write data. Your card can't handles the pattern in some cases.
Your card can handle the sequential write after the file is created.
Here are two SanDisk Extreme cards
SanDisk Extreme Micro SD 16 GB
File size 5 MB
Buffer size 640 bytes
Starting write test, please wait.
write speed and latency
speed,max,min,avg
KB/Sec,usec,usec,usec
275.95,23824,1604,2313
Starting read test, please wait.
read speed and latency
speed,max,min,avg
KB/Sec,usec,usec,usec
427.54,3224,1072,1491
SanDisk Extreme SD 32 GB
ile size 5 MB
Buffer size 640 bytes
Starting write test, please wait.
write speed and latency
speed,max,min,avg
KB/Sec,usec,usec,usec
284.65,12572,1500,2242
Starting read test, please wait.
read speed and latency
speed,max,min,avg
KB/Sec,usec,usec,usec
424.20,3252,1080,1503
Here is a third card:
SanDisk 32 GB Extreme MicroSD
File size 5 MB
Buffer size 640 bytes
Starting write test, please wait.
write speed and latency
speed,max,min,avg
KB/Sec,usec,usec,usec
301.75,12372,1616,2115
Starting read test, please wait.
read speed and latency
speed,max,min,avg
KB/Sec,usec,usec,usec
427.76,3232,1072,1490